1. Field of the Invention
The present invention relates to a method of preparing a carbon cluster film, and more particularly, a carbon cluster film which can stably maintain its electrical conductivity. A carbon cluster film is also described.
2. Background Information
There has recently been proposed a technique of doping by vacuum diffusion of an alkaline metal into a thin film of a carbon cluster such as C.sub.60 or C.sub.70, or so-called "fullerene", consisting of a certain number of carbon atoms linked with each other in the form of spheres such as soccer balls or spheroids such as Rugby balls, as described in articles by R. C. Haddon et al , Nature, Vol. 350, Mar. 28, 1991, pp. 320 to 322 and by A. F. Hebard et al. Nature, Vol. 350, Apr. 18, 1991, pp. 600 to 601. The former article reports that electrical conductivity values of 500 S/cm and 100 S/cm are obtained in C.sub.60 thin films which are doped with potassium (K) and rubidium (Rb) by vacuum diffusion respectively, for example. On the other hand, the latter article reports that a C.sub.60 thin film which is doped with potassium by vacuum diffusion exhibits superconductivity at a critical temperature Tc of 18 K. based on measurements of microwave absorption and magnetization, and at a critical temperature Tc of 16 K. based on a measurement of resistance. It is also reported that a C.sub.60 thin film which is doped with rubidium exhibits superconductivity at a critical temperature Tc of 30 K.
There is such a possibility that a thin film which has an arbitrary conductivity ranging from an insulator to a superconductor can be prepared from the aforementioned carbon cluster by adjusting the amount of a doped alkaline metal. However, a thin film which is doped with an alkaline metal by vacuum diffusion in the aforementioned manner is so unstable in air that its electrical conductivity is reduced in a short time. This deterioration of the conductivity in air is believed to be due to the fact that the alkaline metal itself is highly reactive with oxygen and water as is well known in the art, and a compound formed by reaction with the carbon cluster which is doped with the alkaline metal by vacuum diffusion is so unstable that the same tends to react with oxygen and water contained in the air, for example, to cause decomposition.
In the aforementioned vacuum diffusion, further, it is difficult to precisely control the electrical conductivity even in vacuum since the amount of the alkaline metal which is doped in the thin film cannot be strictly or rather precisely controlled.